The present invention relates to an image converter and an image converting method and in particular to a technique for conducting processes of cutting out a part of a distorted circular image photographed by use of a fisheye lens and converting it into a planar regular image.
A fisheye lens can be used to obtain hemispherical circular images in all directions without using a mechanical moving mechanism. Therefore, a fisheye lens has been extensively used in photographing scenic pictures and others aiming at eccentric effect. However, images photographed by use of a fisheye lens are distorted in circular images, and the images may be used for artistic pictures, as they are, but not suitable for general photographic uses.
Thus, there has been proposed an apparatus for conducting processes of cutting out a part of a distorted circular image photographed by use of a fisheye lens and converting it into a planar regular image. For example, International Patent Publication WO92/21208 under PCT has disclosed technologies by which a computer is used to convert a portion of a distorted circular image to a planar regular image in real time. When this conversion technique is utilized, it is possible to convert a dynamic image made up of distorted circular images photographed by use of a fisheye lens to a dynamic image made up of planar regular images and observe it in real time. Application to a monitoring system and others having a 180-degree field angle is expected.
In order to cut out a portion of a distorted circular image photographed by use of a fisheye lens and convert it to a planar regular image, it is necessary to set parameters indicating, for example, which portion of the distorted circular image is equivalent to a portion to be converted (cut-out position), to which direction the planar regular image displaying the portion is necessary and to which extent the part is to be converted (conversion magnification). As a matter of course, these parameters may be set arbitrarily according to the user's desire.
However, conventional image converters disclosed in the above Patent Document and others have required that the cut-out position and orientation should be designated by three angles called Euler angles and the conversion magnification should also be designated separately. Thereby, such a problem is posed that the operability is poor for a user.
Euler angles are angles that when a dome-shaped virtual sphere is defined on a distorted circular image and a planar regular image-forming face in contact with a desired tangent point on the virtual sphere is defined, a position of the tangent point is expressed by an azimuthal angle α and a zenithal angle β, and an orientation of the planar regular image is expressed by a planar inclination angle ψ. These three angles α, β, ψ are angles which are defined on a three-dimensional space, while a distorted circular image actually seen by a user is an image displayed on a two-dimensional plane. Therefore, an operation of designating the three angles α, β, ψ as parameters is far from an intuitive manipulation for the user.
Another problem of conventional image converters is operation load. Where the above-described Euler angles α, β, ψ are designated as parameters, naturally, the operation requires using transformation equations including these Euler angles α, β, ψ as parameters. However, these transformation equations are complicated equations including trigonometric functions for three angles α, β, ψ, thereby imposing a substantially great operation load to a system. In particular, when an image converter is to be used for an application which requires conversion in real time, high-capacity hardware is required which is capable of trigonometric function operations at a high speed. As a result, there is posed a serious problem in reducing the production cost.